The invention relates to an elevator installation with a number of cars in a shaft, a second car being disposed underneath a first car and there being associated with each car a drive with a traction sheave and also at least one cable strand, which is led over the traction sheave and by means of which the car is connected to a counterweight, the second car being kept in a suspension ratio of 1:1 and being connected to its counterweight by means of two cable strands associated with different sides of the second car and led over the traction sheave of the second car.
The use of a number of cars which can be made to travel up and down separately from each other in a common shaft allows the handling capacity of an elevator installation to be increased. The cars may be driven by means of traction sheaves, over which the cable strands which connect the cars to their counterweight are led. Only a single cable strand is required for the first car. Usually two cable strands are used for the second car, which is disposed underneath the first car, these cable strands being disposed on two different sides of the car and running laterally outside the first car. In U.S. Pat. No. 5,419,414, it is proposed for this purpose to lead the two cable strands of the second car respectively over a separate traction sheave, so that the drive of the second car takes place by means of two traction sheaves which are separated from each other by the width of the car and are coupled to a common drive motor by means of a drive shaft. However, this requires a separate drive motor with a long drive shaft for the second car and also requires a large shaft space, with the effect of increasing the production and operating costs of the elevator installation.
In EP-A-1 329 412, it is proposed to lead the two cable strands of the second car over a common traction sheave. This allows shaft space to be saved and a standard motor to be used for driving the second car. However, the two cable strands of the second car are worn in different ways during its operation, so that they reach their maximum permissible degree of wear at different points in time. To exchange the cable strands, the elevator installation has to be taken out of operation, and, in order to keep these inoperative times as short as possible, the two cable strands are usually exchanged at the same time, although strictly speaking only the more severely worn cable strand needs to be exchanged in this way, while the maximum permissible degree of wear has not yet been reached for the less worn cable strand.
It is an object of the present invention to develop an elevator installation of the type mentioned at the beginning in such a way that, as far as possible, the two cable strands of the second car reach their maximum permissible degree of wear at the same point in time, in order in this way to lower the operating costs.